(a) Field of the Invention
The present invention relates to a hybrid automatic repeat reQuest (HARQ), and more particularly, to a method and an apparatus for performing a synchronous HARQ operation.
(b) Description of the Related Art
In a long term evolution/LTE-advanced (LTE/LTE-A) system, user equipment suffers from an uplink coverage problem due to a short transmission time interval (TTI) and limited transmission power. In particular, when the user equipment is positioned at a cell edge, one Voice over Internet protocol (VoIP) packet may not be transmitted within one TTI (for example, subframe of LTE/LTE-A) due to the limited transmission power but may also be transmitted by radio link control (RLC) segmentation. Therefore, a transmission overhead may be increased and a VoIP service where short delays are stringent may lead to several problems.
Due to these problems, the existing LTE/LTE-A has introduced a TTI bundling scheme. In the TTI bundling applied to an uplink using the synchronous HARQ scheme, instead of segmenting the RLC SDU into smaller units at the RLC layer, one VoIP packet is configured one transport block (TB) and is transmitted in consecutive TTIs without waiting for the hybrid automatic repeat request (HARQ) feedback. After all transmission have been received and decoded by the base station in allocated TTIs, the HARQ feedback is sent.
When the user equipment hits the maximal transmission power, the TTI bundling operation can be activated with higher layer (RRC) signaling based on UE's report. The release of the TTI bundling operation is also controlled by the RRC signaling.
The TTI bundling in the LTE/LTE-A has the fixed bundling size and is applied to only the VoIP traffic. When considering performance requirement at the cell edge of the traffic having various data rates other than the VoIP and the transmission delay shorter than that of the existing LTE/LTE-A system, to decrease the overhead and the delay of the control signal while guaranteeing uplink coverage performance similar to the existing LTE/LTE-A, a scheme of variably allocating transmission time based on scheduling at MAC layer rather than TTI bundling by the RRC signaling is required.
Further, upon the operation of the synchronous HARQ of the LTE/LTE-A system, the HARQ process retransmitted from the receiver side is distinguished without a separate control signal according to the defined HARQ round trip time (RTT). However, assuming same HARQ processing time as the existing LTE/LTE-A, unlike the related art, when the variable transmission time is allocated per HARQ process, collisions between HARQ processes may occur during the retransmission if the allocated transmission times of HARQ process are different. Therefore, the number of retransmission of each HARQ process, etc., may vary according to the allocated transmission time of the HARQ processes. Therefore there is a need to change the synchronous HARQ operation of the existing LTE/LTE-A.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.